The present invention relates generally to optical communications and more particularly to a Variable Line-Rate Optical Transmitter By Modulation Format Hopping Using EOE (electrical-optical-electrical) Generated QAM Signal.
Over the last half decade, the optical carriers have seen exponential growth in network traffics due to bandwidth hungry applications such as data center backhaul, file sharing, cloud computing, high definition video-on-demand, etc. Even as 100 Gb/s/λ systems are being commercially deployed, the growth in bandwidth demand is driving carriers and system vendors to look for technologies beyond 100 G. To achieve high spectral efficiency, future systems will use multi-dimensional modulation formats like quadrature amplitude modulation (QAM) combined with coherent detection and digital signal processors.
Moreover, next generation optical networks will include flexible routing technology such as wavelength selective switches (WSS) to allowing coexistence of mixed line-rates for efficient spectrum usage. In a flexible network management environment, it is desirable to allow these application-specific data channels to dynamically vary its rate according to the application's real-time demand and corresponding link conditions, instead of just using prefixed line-rates like what is done conventionally. When operating at a fixed optical bandwidth, variable must be achieved by dynamically changing signal modulation formats, a challenging task to accomplish with current 100 GbE technologies.
In order to design a transmitter system that supports dynamic change of bit rate, the key challenge is the generation of large quadrature amplitude modulation QAM constellations at high baud rate. In a U.S. patent application no. US 2009/019662 A1, to Saunders et al., digital-to-analog converters (DAC) are used to generate the multi-level signals for optical QAM modulation. Variable line-rate can then be achieved by mapping the signal to different QAM constellation digitally. However, present day high-speed DACs seldom operate beyond ˜10-GHz bandwidth and therefore the teaching of Saunders et al. is impractical for implementing line-rates above 100 Gb/s. Obtaining multi-level electrical signals using passive combination of multiple wide-band binary streams is also very challenging because of unavailability of high-quality broadband electrical components.
Accordingly, there is a need for an improved transmitter that supports dynamic change of bit rate involving a QAM signal.